1. Field of the Invention
This invention relates to a laser apparatus provided with a laser tube, and more particularly to an external mirror type laser apparatus.
2. Description of the Prior Art
As is well known, laser apparatuses provided with a laser tube are widely used for measuring instruments, analyzers, communication apparatuses, image recording apparatuses, image read-out apparatuses, holography, welding apparatuses, medical apparatuses such as laser scalpels, nuclear fusion, and the like.
There are various types of laser apparatuses, one of which is a gas laser apparatus. As the gas laser apparatus, there is known an external mirror type laser apparatus comprising a columnar laser tube and a pair of external mirrors positioned independently of the laser tube along the axis of the laser tube for emitting a laser beam.
In the external mirror type laser apparatus, the external mirrors and mounts for supporting the laser tube for emitting a laser beam are installed on the same supporting plate. In the laser apparatus constructed as described above, when a voltage is applied to the laser tube to emit a laser beam, a very large amount of heat is generated from the vicinity of the middle portion of the laser tube. The heat generated by the laser tube is transferred to the supporting plate via the mounts of the laser tube, or transferred to portions of the supporting plate around the laser tube by heat convection or heat radiation. However, because the amount of heat generated is largest at the middle portion of the laser tube, a difference in temperature occurs between the central portion of the supporting plate where the middle portion of the laser tube is positioned and the peripheral portions of the supporting plate where the external mirrors are positioned. When a temperature gradient occurs through the supporting plate as described above, the external mirrors are shifted out of their positions along the axis of the laser tube due to a difference in the thermal expansion of the supporting plate. As a result, the intensity of the laser beam emitted from the laser apparatus becomes low or, in the worst case, laser emission is not effected. In order to eliminate these problems, it has heretofore been conventional to adjust the positions of the external mirrors periodically, for example, monthly. However, it is very troublesome to correct the positions of the external mirrors and, even when the correction of the positions of the external mirrors is effected, the occurrence of the shift of the external mirrors cannot always be prevented during the use of the laser apparatus.